Wider tires = more rolling resistance. Don't go wider than you actually need and absolutely do not go wider than your rims will support without distorting the sidewalls! I don't recommend any wider than 235 on a 7" rim.
As for the transmission fluid Dexron VI (that's Dex 6 NOT 4) is supposedly cross compatible and has better hydostatic properties and generally runs cooler. I would verify this first though and flush thoroughly before converting. You'll need to further research this though, I haven't dealt with this myself.
A REAL synthetic of either type of fluid will provide more consistent viscosities at various temperatures, definitely a good thing if the car is driven hard.
With a transmission fluid change on a high mileage car I recommend installing an inline filter at the return from the cooler. The fresh detergents are going to free up a lot of crap that's accumulated over the years. Change the filter after a couple months and then once every 10Kmi to 20Kmi thereafter.
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